1. Field of the Invention
The present invention relates to a positioning system, a positioning server, a base station and a terminal location estimation method, and in particular, to a method of easily and accurately detecting a location of a mobile terminal for utilizing a radio communication system.
2. Description of the Prior Art
In recent years, as represented by a portable telephone, services in keeping with a location at which a user exists are provided by utilizing a radio communication system, and these services are generally called location information services.
An example of the location information services will be described by using FIGS. 9 and 10. FIG. 9 is a block diagram showing an example of a configuration of a positioning system in the past. In FIG. 9, reference numeral 21 denotes a positioning server, 200 denotes the Internet, 22 denotes a content server, 23-1 to 23-3 denote base stations, C-1 to C-3 denote cells covered by the base stations 23-1 to 23-3, and 24 denotes a mobile terminal.
The mobile terminal 24 exists in the cell C2, and performs communication with the positioning server 21 and the content server 22 on the Internet 200 via the base station 23-2.
In the case where a user of the terminal 24 obtains information in keeping with an existing location of the terminal 24 (location-related information) such as information on restaurants in the vicinity, a procedure shown in FIG. 10 is applicable, for instance. As shown in FIG. 10, the terminal 24 first sends a location detection request message D1 to the positioning server 21. On receipt of the message D1, the positioning server 21 measures the location of the terminal 24 and sends a location information response message D2 including the location of the terminal 24 to the terminal 24.
As for the location of the terminal 24, it may be the location of the base station to which the terminal 24 is connected, or latitude and longitude at which the terminal 24 exists. The terminal 24 sends a location-related information request message D3 including the location obtained from the positioning server 21 to the content server 22. The content server 22 provides requested information D4 to the terminal 24 based on the request message D3.
In the case of using the location of the base station to which the terminal 24 is connected as the location of the terminal 24, it is rather easy to implement the location information services, and yet the only available accuracy of the location of the terminal 24 is resolution per base station.
As a method of improving the accuracy of the location of the terminal 24, use of a positioning-specific system represented by GPS (Global Positioning System) is known. It is possible, by this method, to obtain detailed information such as latitude and longitude of a terminal location. However, facilities for using the positioning-specific system will be required.
Japanese Patent Application Laid-Open No. 11-275632 shows a method of improving the resolution of the terminal location without using a system such as the GPS. By this method, the terminal measures electric field strengths of the radio waves transmitted from a plurality of base stations, and sends the measurement results to the positioning server having an electric field strength database and a base station database, where the positioning server checks values stored in the above-mentioned databases against the measurement results so as to output the terminal location at a resolution smaller than per base station.
According to the terminal location estimation method shown in Japanese Patent Application Laid-Open No. 11-275632, the location of the terminal to be estimated depends on accuracy of electric field strength measurement of the terminal. As shown in FIG. 11, an area estimated to be equal in the electric field strength of the radio wave transmitted from a base station often becomes a plane instead of a point. In FIG. 11, reference numeral 41 denotes the base station, and 40-1 and 40-2 denote the areas determined to be equal in the electric field strength of the radio wave transmitted from the base station 41 respectively.
For instance, if the area 40-2 is the area which is measured as the electric field strength of the radio wave transmitted from the base station 41=X, it can be determined that the terminal on which the electric field strength of the received radio wave from the base station 41 is measured as X exists in the area 40-2.
The terminal measures the electric field strengths of the received radio waves from a plurality of base stations, and the positioning server estimates the areas in which the terminal exists based on the measurement results and takes the product of the estimated areas. Therefore, the location of the terminal can be estimated at a higher resolution. There is a problem, however, that an resolution of the estimated terminal location becomes rough in the case of a small number of base stations.
In addition, as the terminal measures the electric field strengths of received radio waves from a plurality of base stations, there is a possibility that the measurement may take time and the terminal may move during the measurement in the case where the terminal has only one apparatus for measuring the electric field strength. Although the measurement time can be shortened if the terminal has a plurality of measuring apparatuses, there is a possibility that having the plurality of measuring apparatuses may have an impact on the size, cost and power consumption of the terminal.